Some of liquid crystal display devices, used as display units of mobile phones and personal digital assistants (PDAs), each include a photosensor as a position information input unit.
FIG. 14 illustrates the structure of a PIN diode arranged as a photosensor in such a display device. Referring to FIG. 14, the PIN diode includes a substrate 201, an insulating layer 202 covering the substrate 201, and a semiconductor thin film 203, including a polycrystalline silicon (hereinafter, polysilicon) film, on the insulating layer 202. The semiconductor thin film 203 has a p-doped region (p region) 203p, a lightly p-doped region (i-doped region or i region) 203i, and an n-doped region (n region) 203n in that order. The semiconductor thin film 203 is overlaid with an insulating layer 204. The insulating layer 204 has a contact hole 204a which reaches the p region 203p and another contact hole 204a which reaches the n region 203n. Extraction electrodes 205 are connected to the p region 203p and the n region 203n via the contact holes 204a, respectively. In the photosensor with the above-described structure, the i region 203i serves as a light receiving portion. Pairs of holes and electrons generated by photoelectric conversion in the light receiving portion are extracted from the p region 203p and the n region 203n. 
Such a photosensor is formed in the same step as that of forming a thin film transistor for liquid crystal driving control using a low-temperature polysilicon technique whereby an amorphous silicon film is polycrystallized by laser irradiation to form a polysilicon film. According to the low-temperature polysilicon technique, however, it is difficult to obtain a polysilicon film having a thickness of 80 nm or more. Only part of light coming from above the substrate 201, which is transparent, is absorbed by the polysilicon film formed in the above-described manner, the part being determined by the light absorption spectrum of silicon. Particularly, long-wavelength light ranging from red light to infrared light is hardly absorbed by the polysilicon layer and passes therethrough. Accordingly, it is difficult for the photosensor including such a polysilicon film to obtain an optical signal having a large magnitude.
To solve that problem, the structure in which the surface of the junction (PN junction surface) between the i region 203i and the n region 203n in the semiconductor thin film 203 is inclined relative to the surface of the semiconductor thin film 203 is proposed. In this structure, the increased area of the PN junction allows the semiconductor thin film 203 to more easily absorb light. For example, Japanese Unexamined Patent Application Publication No. 2004-119494 discloses such a structure.
In the structure disclosed in Japanese Unexamined Patent Application Publication No. 2004-119494, however, there is little expectation of significant improvement of the light absorption efficiency at long wavelengths. Furthermore, in this structure, it is difficult to form a photosensor itself. In addition, the angle of inclination of the PN junction surface varies in the range from 10° to 45°, leading to large variations in device characteristics.
Furthermore, when a photosensor with the above-described structure is arranged in a liquid crystal display device with a backlight, a light receiving portion (the i region 203i) of the semiconductor thin film 203 noticeably absorbs noise light coming from the backlight. Accordingly, the ratio of signal component S to noise component N, i.e., the S/N ratio is reduced. Disadvantageously, the sensitivity is not obtained in the application of sensing light coming from above the display device.
Accordingly, it is desirable to provide a photosensor capable of improving the sensitivity to light coming from above a substrate while having uniform device characteristics and a display device including the photosensor.